This invention relates to the production of perfluoro(alkyl vinyl ethers) ethers from xcex1,xcex2-dihaloperfluoro ethers by the elimination of halogen from carbon atoms xcex1 and xcex2 to an ether oxygen.
Perfluoro(alkyl vinyl ethers) are monomers that are widely used in the manufacture of commercial fluoropolymers. A variety of such fluoropolymers are sold by E. I. du Pont de Nemours and Company under the trademark Teflon(copyright) PFA. Perfluoro(alkyl vinyl ethers) can be made by various synthetic routes, but not all routes are suitable for all ethers. In some cases certain ethers can only be synthesized by certain methods, in other cases, reactions are sluggish or incomplete, yields are low, or undesirable byproducts are produced. To overcome these disadvantages, improved synthetic methods are needed.
It is known in organic chemistry that carbon-carbon double bonds can be formed by the elimination of vicinal dihalides (chloride, bromide, or iodide) by treatment with reducing agents such as zinc or magnesium. This synthetic technique has been extended to fluorocarbons, as can be seen by reference to M. Hudlicky, Chemistry of Organic Fluorine Compounds, Ellis Norwood, N.Y., (1992) pp.483-484. This technique has been further extended to the synthesis of perfluoro(alkyl vinyl ethers) by the elimination of halogen from carbon atoms xcex1 and xcex2 to an ether oxygen, as in U.S. Pat. Nos. 4,474,998, 5,350,497 and 5,679,851, and European Patent 0 201 871. These patents teach the use of diglyme, dimethyl formamide (DMF), dioxane, and dimethyl sulfoxide (DMSO), respectively. The corresponding yields are 27%, 40-68%, 85-89%, and 90%. Dehalogenation in dioxane, though in good yield, was carried out at reflux (100xc2x0 C.). For making some perfluoro(alkyl vinyl ethers), reactions in the solvents taught by the references do not consistently proceed at reasonable rates at the low temperatures desirable to minimize side reactions.
The present invention provides a process for the production in good yields and at consistently high reaction rates of perfluoro(alkyl vinyl ethers) from a xcex1,xcex2-dihaloperfluoro ethers by the elimination of halogen from carbon atoms xcex1 and xcex2 to an ether oxygen. In accordance with the present invention, a process is provided for producing perfluoro(alkyl vinyl ethers) represented by the formula Axe2x80x94CFxe2x95x90CFxe2x80x94Oxe2x80x94B comprising the steps of contacting at least one xcex1,xcex2-dihaloperfluoro ether represented by the formula Axe2x80x94CFXxe2x80x94CFXxe2x80x94Oxe2x80x94B with zero valent zinc in the presence of a pyrrolidinone solvent to form a first mixture comprising perfluoro(alkyl vinyl ether) and recovering the perfluoro(alkyl vinyl ether) from the first mixture. In the formulas above:
X is selected from the halogens chlorine, bromine and iodine. X is preferably chlorine.
A is selected from fluorine and normal and branched chain perfluoroalkyl radicals, and B is selected from normal and branched chain perfluoroalkyl radicals. In both A and B, the perfluoroalkyl radicals can contain oxygen and sulfur heteroatoms configured in ether, ester, ketone, and sulfonyl fluoride functional groups. Furthermore, A and B can be bonded together forming a ring. Preferably, A is fluorine and B is selected from perfluoroalkyl radicals containing oxygen and sulfur heteroatoms configured in ether and sulfonyl fluoride functional groups. More preferably, the xcex1,xcex2-dihaloperfluoro ether is selected from CF2XCFXOCF2CF2SO2F and cyclo-(xe2x80x94OCFXCFXOC (CF3)2xe2x80x94) and the perfluoro(alkyl vinyl ether) is selected from CF2=CFOCF2CF2SO2F and cyclo-(xe2x80x94OCFxe2x95x90CFOC (CF3)2xe2x80x94). In the most preferred embodiment, the xcex1,xcex2-dihaloperfluoro ether is CF2ClCFClOCF2CF2SO2F and the perfluoro(alkyl vinyl ether) is CFxe2x95x90CFOCF2CF2SO2F.
xcex1,xcex2-Dihaloperfluoro ethers for use in accordance with the present invention can be prepared by any conventional processes known in the art. For example, preparation of xcex1,xcex2-dihaloperfluoro ether of the formula CF2XCFXOCF2CF2SO2F, where X is chlorine, is disclosed in U.S. Pat. No. 4,962,282. Preparation of xcex1,xcex2-dihaloperfluoro ethers of the formula cyclo-(xe2x80x94OCFXCFXOC (CF3)2xe2x80x94) is disclosed in U.S. Pat. No. 4,535,175. The xcex1,xcex2-dihaloperfluoro ethers are preferably at least 95 wt % pure, and more preferably at least 99.5 wt % pure when employing in the present process. Such purity can be obtained through conventional processes known in the art, such as reduced pressure fractional distillation.
The contacting of the xcex1,xcex2-dihaloperfluoro ethers with zero valent zinc in the presence of pyrrolidinone solvent is preferably carried out at temperatures of about 0xc2x0 C. to 100xc2x0 C., more preferably at about 10xc2x0 C. to about 50xc2x0 C. Most preferably, the contacting is performed at about 20xc2x0 C. to about 40xc2x0 C. It has been discovered that the invention can provide good reaction rates at lower temperature which decreases undesired side reactions. The temperatures for the contacting described above refer to the period during which the bulk of the xcex1,xcex2-dihaloperfluoro ethers are reacted. During the separation of the perfluoro(alkyl vinyl ether) product such as by distillation, some minor amount of unreacted xcex1,xcex2-dihaloperfluoro ethers, if present, may be exposed to higher temperatures. The contacting of xcex1,xcex2-dihaloperfluoro ethers with zero valent zinc preferably occurs in the absence of water and oxygen. Preferably, the contacting occurs in the presence of bromine (Br2). It is believed that bromine activates the zinc surface resulting in reactions with shorter induction periods and higher conversion and yield of perfluoro(alkyl vinyl ether) from xcex1,xcex2-dihaloperfluoro ether. By induction period is meant the period of time between first contact of xcex1,xcex2-dihaloperfluoro ether and zinc, and the generation of a reaction exotherm and sustainable reaction.
Zero valent zinc is preferably finely divided to provide maximal surface area, such as 99.998% 100 mesh zinc powder produced by Aldrich Chemical Co. Inc., Milwaukee, Wis., USA. Such zinc powder may be used as is, or may be activated by processes known in the art which remove surface oxide coatings, such as contact with strong acid such as aqueous HCl followed by rinsing and drying.
When other solvents commonly used in dehalogenation are used such as ethers, alcohols, and organic acids, reaction rates are generally low. The reaction carried out in the presence of a solvent of the pyrrolidinone family consistently gives good yields at good reaction rates. Other solvents may be present provided they do not interfere with the dehalogenation reaction.
The pyrrolidinone solvent preferably is selected from the N-alkyl-2-pyrrolidinones, where the alkyl group may be substituted, and preferably, N-methyl-2-pyrrolidinone (also known as 1-methyl-2-pyrrolidinone). Suitable N-methyl-2-pyrrolidinone is commercially available such as 99+%, A.C.S reagent grade 1-methyl-2-pyrrolidinone offered by Aldrich Chemical Co. Inc., Milwaukee, Wis., USA. Other possible pyrrolidinone solvents available are N-ethyl-2-pyrrolidinone, N-isopropyl-2-pyrrolidinone, N-(2-hydroxyethyl)-2-pyrrolidinone, N-cyclohexyl-2-pyrrolidinone, N-octyl-2-pyrrolidinone, and N-dodecyl-2-pyrrolidinone. The solvent is preferably free of water. Prior to employing in the present process, the solvent may be dried to remove water by conventional drying agents such as molecular sieves or metal hydrides such as CaH2 and may be further purified by distillation.
The recovering of perfluoro(alkyl vinyl ether) from the first mixture may be carried out by extraction or distillation. Preferably, distillation of the first mixture is employed and more preferably, reduced pressure distillation of the first mixture is employed which affords high yields of perfluoro(alkyl vinyl ether) as distillate.